Radiation units summary
Zack Lasner
June 13, 2016
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Radiation units
1.1 Activity
Activity is the number of radioactive events per second, typically given in becquerel (Bq) or curie (Ci). 1 Bq is dened as 1 event per second and 1 Ci is
dened to be 37 GBq (approximately the activity of 1 g of 226 Ra):
1 Bq =
1 events/s
1 Ci = 3.7 × 1010 events/s
1 Ci
=
37 GBq
1.2 Absorbed dose
The dose a material receives is the energy it absorbs from a radioactive source
per unit mass of the dosed material. Common units of absorbed dose are the
gray (Gy) and rad, dened as follows:
1 Gy =
1 rad =
1 Gy
1 J/kg
100 ergs/g
=
100 rads
1.3 Exposure
Exposure characterizes the level of radioactivity in some area independently of
the material the radiation is incident on, unlike absorbed dose. One exposure
unit (X unit) is the quantity of X- or gamma radiation that ionizes 1 C of
total charge in a kilogram of air. The more commonly used exposure unit is
the roentgen (R), dened to ionize 1 statcoulomb of total charge in a cubic
centimeter of standard air:
1 X unit =
1 C/kg air
3
1R
= 1 SC/cm air
1 X unit
=
3881 R
1
To convert between from exposure in R to absorbed dose in rads for a general
material m, the exposure must be scaled by the ratio 0.877(µ/ρ)m /(µ/ρ)air , as
in Cember Eq. 6.12. Here µ is the energy absorption coecient (the inverse of
the absorption length) and ρ is the material density. In general, µ depends on
the radiation type and energy.
1.4 Equivalent dose
Dose equivalent normalizes each type of radiation to its propensity to cause
biological damage, or its relative biological eectiveness (RBE). It is computed
as
Dose equivalent H = absorbed dose D ×quality factor Q.
Quality factors for dierent kinds of radiation can be found in Table 7.3 in
Cember's Introduction to Health Physics. For x-rays, gamma rays, and beta
rays, Q = 1; for alpha particles, Q = 20. The SI unit of equivalent dose is
the sievert (Sv), dened to be the dose equivalent of 1 Gy for radiation with a
quality factor Q = 1. The more common unit is the roentgen equivalent in man
(rem), dened to be 0.01 Sv so that 1 roentgen deposits approximately 1 rem
in human biological soft tissue:
1 Sv
1 rem
=
=
1 Gy with Q = 1
0.01 Sv
1 rem
1 rem
↔
1 rad (x-rays)
↔ 0.877 R (x-rays, air)
The last two lines convert rem (equivalent dose) to rad (absorbed dose) and
roentgen (exposure), respectively.
1.5 Eective dose
Eective dose accounts for the nonuniform sensitivity of dierent parts of the
body by including a weighting factor for each area according to Table 1 (taken
from Table 8.1 in Cember).
The total eective dose is the weighted sum of local doses. When the entire
body uniformly receives the same dose, the total eective dose reduces to the
dose to each area of the body.
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Occupational limits
Occupational limits are dened in terms of eective doses. Radiation attenuates
as it penetrates the body, so the eective dose to each kind of tissue is computed
for an appropriate attenuation length. The following categories are relevant:
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Tissue
Gonads
Breast
Red bone marrow
Lung
Thyroid
Bone surface
Remainder
weight
0.25
0.15
0.12
0.12
0.03
0.03
0.30
Table 1: Tissue weighting factors
• Deep dose equivalent (DDE): dose equivalent at 1 cm penetration of soft
tissue
• Lens dose equivalent (LDE): dose equivalent at 0.3 cm penetration into
the eye
• Shallow dose equivalent (SDE): dose equivalent at 0.007 cm penetration
(beneath dead skin)
• Total eective dose equivalent (TEDE): computed only from DDE in the
case of no internal sources (e.g., from ingestion)
The annual occupational limits for radiation workers are as follows:
TEDE
LDE
DDE for any single tissue
SDE for skin and each extremity
< 5 rem
< 15 rem
< 50 rem
< 50 rem
The limit on total eective dose equivalent is smaller than the other limits
because it incorporates the tissue weighting factors. For example, a deep dose
equivalent of 50 rem to the thyroid is only an eective deep dose equivalent of
50 rem × 0.03 = 1.5 rem.
These limits are only to provide a general sense of scale; it is unacceptable
for members of the ACME experiment to receive doses approaching the legal
occupational limits. As another reference, the background level of radiation is
approximately 300 mrem.
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